Gas chromatography (GC) is used to separate solutes or components of an analyte sample for measurement. Generally, a GC device includes a downstream column system, including one or more capillary columns for separating the solutes. The columns are typically made of metal, glass or quartz, for example, and coated on the inside with a thin-film coating or stationary phase. The GC process includes mixing the analyte sample with a carrier gas, such as hydrogen or helium, and introducing the sample/carrier gas mixture into the column(s) using a continuous flow. Various solutes within the sample react differently with the stationary phase, and thus move at different speeds through the column(s), resulting in separation of the solutes. The separated solutes may then be detected by various detectors or provided as input to a mass spectrometer (MS) device, for example. An implementation in which the GC device provides samples in gaseous form to an inlet of an MS device is a gas chromatography mass spectrometer (GC-MS).
Conventional GC devices may include an electronic pressure control (EPC) system, which automatically controls carrier gas flow based on pressure sensing, for reproducible sample mixture injections at an inlet of the column(s), enabling chromatographic separation along the column(s) and detection upon the sample exiting the downstream column system. In order to achieve high column resolution and detector sensitivity, as well as short analysis time, the constant mass flow rate through the downstream column system is controlled to be a constant value.
For example, in a calculated pressure programming method, an example of which is described by KLEIN et al., in U.S. Pat. No. 4,994,096 (Feb. 19, 1991), which is hereby incorporated by reference, the pressure at a column inlet is controlled dynamically according to its relation with column mass flow rate, column dimension (e.g., length and inner diameter) and a temperature program to which the column subjected, given a particular type of carrier gas. However, the calculated pressure programming method is difficult to implement when the precise dimensions of the downstream column are not known, the downstream column includes multiple segments that are independently or temperature programmed, and/or the downstream column has additional flow or pressure control points before the detector to effect advanced functions, such as midpoint concurrent backflush and multidimensional GC, also known as heart cutting.
In addition, the calculated pressure programming method is difficult to implement when the downstream column does not feature a well-defined round cross-section. For example, when the column is not a drawn fused silica capillary, but rather, is microfabricated on silicon or other substrate, it may feature rectangular, trapezoidal or other variant cross-sectional geometry, and/or may have a serpentine layout to maximize usage of a small chip area, which contribute to a sophisticated and generally unknown relation to permeability exhibited by the column. Likewise, the calculated pressure programming method is difficult to implement when the column is subject to thermal control that is not fully integrated into the GC device, so that the temperature program of the column is unknown and/or difficult to monitor. An example of an unknown and/or difficult to monitor temperature program is a low thermal mass (LTM) column module that directly heats the column without a conventional GC oven, an example of which is described by MUSTACICH, et al. in U.S. Pat. No. 6,682,699 (Jan. 27, 2004), which is hereby incorporated by reference.